The fastener manufacturing industry has had the problem of making metal fasteners possessing at once characteristics of ductility and toughness, yet having relative thread hardness.
By being ductile or tough, a metal is considered herein as being pliant yet strong, as having the quality of bending or twisting without tearing or breaking. Antonymously, "hardness" is used herein to characterize increasing susceptibility to breaking, or brittleness, in addition to resistance to penetration. In the art or trade, it is customary to use the "Rockwell" hardness testing devices. Using such devices, a metal is penetrated by a substance of known hardness, such as a diamond, with a predetermined force. The depth of penetration gives a relative indication of hardness. A number is assigned directly relative to the penetration depth. The results of such a device are ordinarily given on the "Rockwell" scales designated A, B and C. The higher the number assigned as a result of such a test, the harder the metal. Thus, a metal having a hardness corresponding to a reading of C = 30 or less on the Rockwell scale would be of the order as is normally considered ductile and is used in the manufacture of relatively tough, ductile fasteners.
A fastener made from such tough, ductile material generally fails as a self-tapping screw. A self-tapping screw is one which forms its own mating threads or reciprocal grooves in a drilled hole in metal or in a hole into which the fastener is being inserted. Failure occurs when the ductile or tough thread on the fastener collapses within the unthreaded hole.
Various methods are presently used to make the fastener harder on its outside surface while maintaining toughness. The most common, presently used process is a three-step process. First, the fastener is heat treated to approximately 1700.degree. F or more. The fastener is then in a second step case hardened. In this second step, the fastener's surface is thoroughly cleaned, heated to approximately 1250.degree. F to 1300.degree. F and placed into a carbon rich atmosphere. In such an environment, the metal has a propensity to attract the carbon. The usual result is an approximately 0.004 inch to 0.006 inch carbon layer deposited upon the fastener's surface. The third procedural step is entitled induction heat treating. In this step, the pilot end of the threaded fastener is placed in a rapidly changing inductance field. The iron in the metal has its magnetic field changed by each hysteresis cycle, and thus is heated very rapidly to red hot temperatures. Upon cooling, the molecular structure at least of approximately 0.06 inch of the exposed surface of the pilot end is changed. The result of such treatments, unfortunately, hardens the fastener and makes it more brittle. In fact, such an induction heating procedure has made test fasteners possess a hardness on the order of C = 45 and higher on the aforementioned Rockwell scale.
To manufacture these relatively hard fasteners in the currently known manner, a premium alloy steel containing an additional metal alloy ingredient is required. Such additional metal could be manganese, chromium, nickel and the like except carbon. These alloys are expensive. Furthermore, the three-step procedure outlined above requires careful attention and handling. Prior to case hardening, for example, the fasteners and the thread root area between the threads must be thoroughly cleaned of the lubricating oils and other grit of manufacture so that the carbon will evenly and finely deposit upon the surface of the fastener. The carbon rich atmosphere must be precise and requires careful attention.
Additionally, the induction-heating step requires meticulous positioning of the fasteners, lest the entire fastener be so treated. Moreover, the equipment for the induction-heating step is complex and very costly.
The result of all of this trouble and expense is a fastener which has substantially lost the desired ductility and toughness, and in some instances has not achieved the desired hardness. For example, a fastener having a Rockwell C = 45 measure of hardness would be too hard and brittle in certain applications where predictable shocks require the fastener to have a certain resilience, ductility and toughness. Yet, the threads of such a fastener even after case hardening and induction treating have collapsed when attempts were made to tap a hole.
The ability to make threaded screw fasteners made of comparatively tough, resilient material possessing threads having stronger, hard characteristics has been long sought, but heretofore has been unavailable.